avfilter/fade: use AV_OPT_TYPE_BOOL for alpha option

libavfilter/vf_fade.c

@@ -384,7 +384,7 @@ static const AVOption fade_options[] = {
                                                     OFFSET(nb_frames),   AV_OPT_TYPE_INT, { .i64 = 25 }, 0, INT_MAX, FLAGS },
     { "n",           "Number of frames to which the effect should be applied.",
                                                     OFFSET(nb_frames),   AV_OPT_TYPE_INT, { .i64 = 25 }, 0, INT_MAX, FLAGS },
-    { "alpha",       "fade alpha if it is available on the input", OFFSET(alpha),       AV_OPT_TYPE_INT, {.i64 = 0    }, 0,       1, FLAGS },
+    { "alpha",       "fade alpha if it is available on the input", OFFSET(alpha),       AV_OPT_TYPE_BOOL, {.i64 = 0    }, 0,       1, FLAGS },
     { "start_time",  "Number of seconds of the beginning of the effect.",
                                                     OFFSET(start_time),  AV_OPT_TYPE_DURATION, {.i64 = 0. }, 0, INT32_MAX, FLAGS },
     { "st",          "Number of seconds of the beginning of the effect.",